[MlMt] Searching and switching mailboxes

[Seebs]

Enter mailbox A, create a search, go to look in mailbox B, cancel 
search, and you're back in mailbox A.

I think this is a bug, but I'm not sure.  (My intuition is that 
switching mailboxes intrinsically cancels a search, but I think I'd be 
fine with the search staying active, and even prefer it; it's just that 
clearing the search by hitting the - shouldn't change mailbox 
selection.)

-s